Field of the Invention
The present invention relates to a method for manufacturing insulated gate bipolar transistor (IGBT), especially to a low-temperature epitaxial method for manufacturing backside field stop layer of IGBT.
Description of Prior Art
Insulated gate bipolar transistor (IGBT) is a semiconductor device combining the features of metal-oxide-semiconductor field effect transistor (MOSFET) and bipolar junction transistor (BJT). IGBT has the advantages of gate-controlling ability of MOSFET and low turn-on voltage of BJT and is thus extensively used in high voltage and high power applications.
A conventional IGBT, such as a punch through (PT) IGBT, mainly comprises a P+ semiconductor substrate and N− buffer layer arranged thereon. An N epitaxial layer is formed on the N− buffer layer and functions as drain of parasitic MOSFET in the IGBT. Moreover, a gate and an emitter are formed in the N epitaxial layer, and a collector is formed on bottom face of the P+ substrate. The breakdown voltage of the PT IGBT is determined by the P+ substrate and N− buffer layer because a maximum electric field is generated therebetween.
Another conventional IGBT, such as a non-punch through (NPT) IGBT, does not use N− buffer layer. The breakdown voltage is determined by the avalanche mechanism of the N epitaxial (N drift) layer. To increase the breakdown voltage, field stop (FS) IGBT was developed where the N− buffer layer in PT IGBT is replaced by a field stop ion implantation layer. Therefore, the abrupt junction in the PT IGBT is also replaced by a graded (such as a linearly graded) N type profile. The value of the maximum electric field can be advantageously reduced to enhance breakdown voltage.
In the conventional IGBT with field stop layer, the backside field stop layer is needed to be fabricated on backside of the device before forming the front side electrode (generally electrode containing aluminum). The aluminum has melting point around 630 centigrade degree, while the thermal driving-in step after the ion implantation for the field stop layer needs high temperature of 900 centigrade degree, which will damage the aluminum-based electrode. However, in conventional process for IGBT with field stop layer, protection layer is first used to protect the front side of the IGBT semi-finished product, which has not the front side electrode, and then the backside field stop layer is fabricated. Therefore, the manufacture process is complicated.